The present invention relates to a method for manufacturing a so-called after-recording or DRAW (Direct Read After Write) type optical disk on which the user can write data, especially, an optical disk with address pits and guide grooves.
An example of an after-recording type optical disk is shown in FIG. 1, which is a sectional view with parts cut away. In FIG. 1, reference character A designates a spacer; B, a space; C, a substrate; D, a recording material; E, a data part (user bits); F, an address part in which track addresses, etc., are recorded; and J, guide grooves. The pits in the address part F (hereinafter referred to as address pits) and the guide grooves are formed by the manufacturer, while the user pits are recorded by the user. In the area of the user pits in FIG. 1, reference character H designates a region in which no data has been recorded, and G, recorded pits.
In the optical disk with addresses and guide grooves, in order to maximize tracking signals, in general, the guide groove depth is set to .lambda./8n (where .lambda. is the wavelength of a recording and reproducing light beam and n is the refractive index of the substrate) while the address bit is set to .lambda./4n so that the reflected light is maximum in contrast.
Heretofore, an optical disk of this type has been manufactured as follows: As shown in FIG. 2, a light beam from a laser beam source 1, which is a photoresist exposing light source, is modulated by an electrical and optical modulator (hereinafter referred to as an E/O modulator) 2, and the modulated beam is reflected by a reflecting mirror towards a focusing lens 3.
In FIG. 2, reference numeral 4 designates a recording original board made of glass or the like the surface of which is polished; 5, a photoresist layer having a thickness of about .lambda./4; and 6, an electric motor for rotating the recording original board 4.
FIG. 3 is a graph indicating amounts of exposure to the photoresist layer 5 with resulting percentage of film (photoresist material) remaining after developing. With the amount of exposure L, the percentage of film remaining is 0%. With the amount of exposure K, the percentage of film remaining is 50%.
When the recording light beam (light spot), after being modulated to a light output corresponding to the amount of exposure K for the guide groove J and to a light output corresponding to the amount of exposure L for the address part F, is applied to the recording original board 4 as shown in FIG. 4, an optical disk is produced with addresses and guide grooves in which the guide groove depth is one-half of the address pit depth.
In the above-described method, with the amount of exposure L with which the percentage of the photoresist layer 5 remaining is 0%, the formed groove has a flat bottom as shown in FIG. 5B. However, since the light source is a laser beam having a Gaussian distribution, the method is disadvantageous in that, even if the amount of exposure K is provided for the center of the light spot is proper, due to the unavoidable intensity decrease radially of the center, the guide groove formed upon developing is actually round in section, as shown in FIG. 5A.
Furthermore, when the amount of exposure is slightly changed rom the value K, the percentage of film remaining is greatly changed, as a result of which it is difficult to control the guide groove's depth with high accuracy.
If the section has a width W.sub.A at half of the maximum depth when the amount of exposure K is employed and a width W.sub.B when the amount of exposure L is employed, the width W.sub.B is smaller than the width W.sub.A at all times (W.sub.A &lt;W.sub.B). Therefore, it is impossible to form a guide groove in which the upper part is smaller in width than the lower part.